Utilizing Field Data to Validate Finite Element Analysis Modeling of a Coiled Tubing Intervention Stack on a North Slope of Alaska Well
- Kenneth Ray Newman (NOV CTES) | David Allen Traugott (NOV CTES) | Steven L. Deckert (BP Exploration Alaska Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE/ICoTA Coiled Tubing & Well Intervention Conference and Exhibition, 31 March-1 April, The Woodlands, Texas
- Publication Date
- Document Type
- Conference Paper
- 2009. Society of Petroleum Engineers
- 1.7.5 Well Control, 4.5 Offshore Facilities and Subsea Systems, 3 Production and Well Operations, 1.10 Drilling Equipment, 4.3.1 Hydrates, 3.2.2 Downhole intervention and remediation (including wireline and coiled tubing), 1.6 Drilling Operations
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BP Exploration in Alaska were planning Coiled Tubing (CT) operations on wells that have components in the wellhead stack with 2-9/16??, 5,000 psi components at the bottom of the stack. There was a concern that the CT intervention stack and associated equipment could damage these small wellhead components during the well intervention process.
A finite-element analysis (FEA) was performed of the intervention stack, followed by a field test in which a bending moment and force measuring gauge was used to measure the actual moments and forces being applied. Results from the field test were used to improve the FEA modeling. This paper presents the results from the modeling and the field test, and discusses the conclusions from this study.
On the North Slope of Alaska, large CT units like the one shown in Figure 1 are used to perform interventions. These units use a mast with a T-bar to support the injector and well control stack on the wellhead. In some cases the wells have dual completions, with small components in the wellhead. The particular wellhead of interest for this study is shown in Figure 2. The tubing head adapter was the primary concern. It had a maximum bending moment specified by the manufacturer of 14,400 ft lb. The other 2-9/16?? 5,000 psi components in the wellhead had higher bending limits, but were still a concern.
In an effort to understand the forces and bending moments associated with utilizing a T-bar structure to support the stack, a FEA was performed using a proprietary software package, known as the Zeta1 Model. The preliminary results from the analysis indicated that, if the CT injector was held close to the centerline of the wellhead and the injector was not allowed to rotate (tilt) toward the reel when reel back tension (RBT) was applied, the existing intervention stack design was within allowable load limits. However, if the T-bar allowed some tilting of the CT injector due to RBT, or if the injector was not closely-aligned with the wellhead (there was some lateral displacement), the bending moment on the tubing head adapter could exceed the manufacturer specified limit.
It was decided to perform a field test to mitigate the uncertainty associated with flexibility of the T-bar support and the flexibility of a quick connect in the well control stack. A bending moment and force measuring device, known as a Zeta1 Gauge, was run in the well control stack to provide real-time measurements during a field job. This job was performed on a wellhead with larger components to avoid a possible failure during the test. The field test answered the questions regarding rigidity of the T-bar support and flexibility of the quick connect. Once the model was updated to include these results, the field data agreed well with the pre-job modeling results. The model could then be rerun for the primary case of interest which contained the smaller wellhead components. It was determined that interventions incorporating the smaller wellhead components could be run safely with careful attention being paid to the alignment of the injector and well control stack with the wellhead.
The Zeta Model utilized to perform this intervention stack analysis is a purpose written 3D nonlinear finite-element analysis model. The FEA analysis is run repeatedly at very small time increments to perform the dynamic analysis. A finite difference, forward difference scheme is utilized to determine the acceleration of each component in the stack, and the associated dynamic forces. Although in this case the stack is static, the dynamic calculation is needed to accurately determine the buckling load.
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